Welding repair method for full hoop structures

ABSTRACT

A unique heat treat method for relieving stresses caused by a repairing weld joint in a full hoop part heat treats locally, at the location of the weld joint, and at a diametrically opposed location. By providing the diametrically opposed heat treat location, the present invention relieves stresses caused by the weld joint, without creating any additional residual stress in the weld joint.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/208,801, which was filed Aug. 22, 2005.

BACKGROUND

This application relates to a method of repairing a full hoop structurewith a welding process, wherein heat treating is performed both at thelocation of the weld, and at a diametrically opposed location.

Welding methods are sometimes necessary to repair metallic structures.As an example, a cast part may have a defect such as shrinkage that mayhave occurred in a cast mold. Alternatively, small cracks may form inthe part.

Such operations are often used in components for gas turbine engines.Structures that could be called “full hoop,” or structures that surrounda central axis for 360°, often require such welding procedures. Examplesof such parts in a gas turbine engine would be a diffuser case and aturbine exhaust case.

The weld being performed at a location on the part may cause anunacceptably high residual stress. In the prior art, this stress hasbeen relieved by some post-weld heat treatment.

In one prior art method, the entire structure has been heatedisothermally to heat-treat temperatures. Heating isothermally does notinduce additional thermal stress at the weld, so the residual stressremains constant until actual heat treatment takes place. This “global”heating can affect dimensions that have been “machined” into the part bycausing their residual stresses to also relax. In many cases, it has notbeen found practical due to cost and complexity to fixture the partduring heat treatment to hold these dimensions constant.

Thus, localized heat treatment has also been utilized to avoid loss ofdimensions. Local heat treatment can have unforeseen and potentiallydetrimental effects on the intended stress relaxation. The region beingheated locally will expand due to its temperature change. Thesurrounding non-heated material will resist this expansion causing theheated area to become more compressively loaded. Since the residualstress due to weld is tensile, the net effect of local heating is totemporarily reduce the value of the tensile stress in the weld. Ifsufficient care is not exercised, it is possible to reduce the value ofthe tensile stress so much so as to eliminate it completely. In thiscase, subsequent heat treatment for stress relaxation would beineffective since the stress would already be reduced to zero. Note thatthe full value of the residual stress in this case would return when thelocally applied temperature was removed.

Also of concern, would be a situation in which the weld stress wasreduced by local heating through zero and into a state of compression.This stress would relax during subsequent heat treatment, but this isfar from the original intent of the heat treatment process, which was toreduce the tensile residual stress associated with the weld.

SUMMARY

In the disclosed embodiment of this invention, a weld repair is made ona part with a full hoop structure. After the weld has been completed,heat-treating is performed at the location of the weld, and at the sametime, at a second opposed location. In a disclosed embodiment, thesecond location is diametrically opposed to the weld location. Thisheat-treating is preferably confined to as narrow a band as possiblethrough the weld and its heat affected zone, and in a similar manner, atan opposed position to it. Furthermore, the heat-treating preferablyoccurs along an entire axial length of the part.

The opposed bands of heat-treating eliminate the compressive stressesmentioned above from forming. This allows the modified local heattreatment to mimic the beneficial effect of a global heat treatment asmentioned above while avoiding the inherent problems.

While in the disclosed embodiment the part is a full hoop part, thepresent invention is more powerful, and extends beyond any particularshape of part. In fact, an arbitrarily shaped part could benefit fromthis present invention. In an arbitrarily shaped part, an area ofmaterial on the part would be identified about which the part wouldthermally expand while not creating additional stress in the part at aweld treatment location. The weld treatment would be provided, andsimultaneously, a local heat treatment would be provided at an area ofthe weld treatment, and at the identified area.

In other optional embodiments, the second band could be a plurality ofbands, which are displaced from the diametrically opposed location. Asan example, two separate bands spaced equally about a location spaced180° from the weld treatment area could be utilized rather than a singlesecond band.

In yet another embodiment, the second band can extend for a greatercircumferential extent than the band about the weld treatment. In thismanner, the heat treating on the second band can be at a lowertemperature. By utilizing a lower temperature, the potential resultantdimensional changes in that second region can be reduced. Suchdimensional changes are related to temperature, and thus being able toutilize a lower temperature, albeit over a larger area, might provebeneficial under certain applications.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a gas turbine engine.

FIG. 2 is a schematic view of a full hoop part.

FIG. 3 is a cross-sectional view of a heat treatment occurring on thementioned part.

FIG. 4 shows yet another embodiment.

FIG. 5 shows yet another embodiment.

DETAILED DESCRIPTION

A gas turbine engine 10 is illustrated in FIG. 1 extending along anaxial center line 12. A fan section 14 is upstream of a compressorsection 16, a combustion section 18, and a turbine section 11. As isknown, many components of a gas turbine engine 10 could be said to havea “full hoop” structure. The full hoop is defined as a structure thatsurrounds the axial center line 12 for 360°. An example of such fullhoop structures found in the gas turbine engine 10 would include adiffuser case located downstream of the compressor, or a turbine exhaustcase located downstream of the turbine section 11. The term “full hoop”should not be taken as requiring that the component would becylindrical. In fact, the disclosed components could be better describedas somewhat conical. Even that shape is not a limitation on thedefinition of “full hoop” which could extend to non-symmetricalstructures, or structures with complex surfaces and multi-faceted shapesat their outer surfaces.

As shown in FIG. 2, such a part 50 can have defects such as a crackshown at 52. Other type defects may be a casting defect such as may becaused by shrinkage. A worker of ordinary skill in the art wouldrecognize many of the known defects, which could require welding repairtreatment.

As shown in FIG. 3, a weld treatment 53 is being applied schematicallyby welding tool 60 at the crack 52. With the present invention, andafter completion of the welding treatment, two narrow bands of heattreatment are applied at diametrically opposed locations 54 and 56.Preferably the circumferential extent of the bands is selected to onlybe wide enough to provide the stress relief at the weld joint 53 alongthe defect 52. Thus, the bands may well have the same circumferentialwidth. As shown, heating structures 58 create these two heat treatlocations. The heating structures may be induction coils, radiant lamps,gas burners, etc. The heat treatment can be on the order of 1500° F.,although the heat treat temperatures may be as known in the art. Thebands 54 and 56 extend along the entire length of the part 50, as shownin FIG. 2. Of course, it may also be that the bands do not extend forthe entire length of the part.

The present invention, by utilizing the two diametrically opposed bands,achieves the benefits provided by the global heating of the prior art,but also avoids the problems of global heating as encountered in theprior art.

Also, while the present invention is disclosed as being directed to fullhoop parts, it would have benefits in certain parts that do not have thefull hoop structure as defined above. Arbitrarily shaped parts couldbenefit from the present invention by heat treating two distinct zones,to allow the numerical value of weld residual stress to be heat treated,while greatly reducing or eliminating the liability of resultantdimensional changes. For non-full hoop structures, a line or plane ofmaterial to be locally heat treated as the second band, is the line orplane about which the structure would thermally expand without creatingadditional stress in the component at the weld. A worker of ordinaryskill in this art can determine this line or plane with structuralanalysis.

FIG. 4 shows another embodiment wherein the “second band” is actuallyprovided by two separate bands 202 and 204. As can be appreciated, thetwo separate bands are disclosed as being spaced equally about the pointP spaced 180° from the weld treatment area T. By positioning theseseparate bands about the point P, the beneficial effects provided by theabove-disclosed embodiment can be achieved.

FIG. 5 shows yet another embodiment wherein the circumferential extentof the second band 300 is wider than the circumferential extent of theweld band 302. The temperature provided at the second band 300 can belower, such that potential resultant dimensional changes in this secondband are reduced.

Again, a worker of ordinary skill in the art would recognize how toincorporate the optional embodiments of FIGS. 4 and 5 to best effect.

Although an embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

1. A method of providing a weld treatment on a part comprising the stepsof: (a) identifying a part in need of a weld treatment to correct adefect; (b) identifying an area of material on the part about which thepart would thermally expand while not creating additional stress in thearbitrarily shaped part at the weld treatment; (c) providing the weldtreatment; and (d) simultaneously providing a local heat treatment at anarea receiving the weld treatment, and at the identified area.
 2. Themethod as set forth in claim 1, wherein the part is a full hoop partsurrounding a central axis by 360°.